1. Field of the Invention
This invention relates to a process for producing a thermoplastic polymer sheet. More specifically, it relates to a process for producing a thermoplastic polymer sheet having excellent surface quality and properties by extruding a molten thermoplastic polymer onto the surface of a cooled moving body while bringing them into intimate adhesion to each other.
2. Description of the Prior Art
Generally, thermoplastic polymer sheets are produced by extruding a molten thermoplastic polymer from an extrusion die, casting the polymer over the surface of a forcibly cooled moving body thereby to cool and solidify the polymer as a sheet, and then peeling off the sheet from the surface.
In the present specification, the "cooled moving body" will be referred to simply as a "cooling body" in the sense that it serves to cool a molten thermoplastic polymer which has been extruded in sheet form.
Such a process, however, involves various difficulties, and it is practically impossible to form polymer sheets of the desired quality by this process. First of all, the molten thermoplastic polymer frequently slips on the surface of the cooling body, and "creases" tend to occur on the surface of the cooled and solidified polymer sheet. Secondly, even when slipping does not occur, rapid cooling of the molten polymer on the surface of the cooling body causes a reduction in sheet width, and the resulting sheet tends to have an irregular non-uniform width. Thirdly, air is irregularly entrapped between the molten polymer and the cooling body, and the polymer sheet tends to have raised and depressed portions or an unevenness due to cooling. Fourthly, since the points at which the molten polymer contacts the cooling body are not stable and varyirregularly, an unevenness in thickness or an unevenness due to cooling tends to occur in the polymer sheet. These defects increase as the moving speed of the cooling body is increased to obtain increased output of polymer sheets.
The defects of the prior art have been considered to be due to the insufficient adhesion between the molten polymer and the cooling body, and various attempts have been made to increase the adhesion.
A first attempt was to blow air onto the surface of a molten thermoplastic polymer that has been extruded, or to press the extruded polymer mechanically against a cooling body by means of a press-adhering drum. The former attempt, however, has the defect of restricting the surface properties of the polymer sheet due to disturbance by the air stream, changes in the air pressure, etc.
A second attempt was to use a method which comprises generating an electrostatic charge on the surface of a molten thermoplastic polymer which is to contact the cooling body or on the opposite surface thereof, or on the electrically insulation-coated surface of the cooling body, and simultaneously grounding the cooling body, thereby to generate an electrostatically attractive force due to the difference in the polarity of the electrostatic charge between the molten polymer and the cooling body and to adhere the molten polymer intimately to the cooling body [see, for example, Japanese Patent Publication Nos. 6142/62 (corresponding to U.S. Pat. No. 3,223,757), 3535/73, 14786/73, 14785/73, and 29311/73].
These methods can eliminate almost all of the defects of the prior art described above. However, due to a localized concentration of electrostatic charges, an abruptly increased force is exerted in localized areas in the molten polymer. The polymer is thus subjected to an abrupt change in stress in these areas, and creases or an unevenness in thickness tends to be formed in the polymer sheet. Since the adhesion using this method is too strong, imperfections on the surface of the cooling body are transferred to the polymer sheet. The electrostatic charge remaining after the adhesion, on the other hand, may cause the adhesion of dirt and dust, and specks tend to appear locally on the resulting polymer sheet. When the moving speed of the cooling body is increased in order to increase the output of the polymer sheets, the molten polymer extrudate vibrates between the extrusion die and the cooling body. This causes additional defects such as periodic changes in the width of polymer sheet or the occurrence of an unevenness due to cooling. Hence, neither of these methods provides polymer sheets having an entirely satisfactory quality.
A third attempt includes various methods in which a liquid is interposed between the molten polymer and the cooling body. One such method is disclosed in British Pat. No. 1,312,519. This method uses the electrostatic method described above and simultaneously involves providing an ultrathin film of a heat-conducting liquid on the surface of a cooling body and extruding the molten polymer onto the cooling body thereby to eliminate the defect in which the surface imperfections of the cooling body which are caused by the excessively strong adhesion of the electrostatic method are transferred to the polymer sheet.
In the method of British Pat. No. 1,312,519, the ultrathin film of the heat-conducting liquid on the cooling body such as a cooling roll should be completely removed from the surface thereof as a result of evaporation or of absorption by the polymer before the cooling body is reused (before the cooling body has rotated once, in the case of a cooling roller). The liquid film should be extremely thin since the amount of the liquid film removed is generally very small. The liquid film thickness, which varies according to, for example, the type of thermoplastic polymer or the temperature of the cooling body, should generally be less than several tens of microns. When the polymer is polyethylene terephthalate, the liquid film thickness should be within the range of about 0.5 to about 10 microns. In general, however, it is extremely difficult to provide such an ultrathin film on the surface of a cooling body that is moving, and it is almost impossible to adjust the liquid film thickness to the optimum value for a given thermoplastic polymer, or to control the liquid film thickness in response to changes in the manufacturing conditions. In practice, therefore, a thermoplastic polymer sheet of the desired quality is difficult to obtain using such a method.
The method disclosed in Japanese Patent Application (OPI) No. 99160/74 appears to be based on the same idea as that in this British Patent. This method comprises coating an alcohol having a boiling point of as high as about 150.degree. to about 220.degree. C. and a surface tension at room temperature (about 20.degree.-30.degree. C.) of not more than about 50 dynes/cm on the surface of a cooling body, and extruding a linear polyester resin in the molten state onto the surface of the alcohol coating on the cooling body thereby to cool and solidify the resin. Since the alcohols used in this method do not easily evaporate because of their high boiling points, the amount of the alcohol to be consumed by evaporation is small. Thus, just as in the method of the British Pat. No. 1,312,519, the thickness of the alcohol film should be reduced markedly, and in practice, great difficulty is encountered when this method is used.
A further attempt is disclosed in British Pat. No. 1,140,175. This patent discloses a technique in which molten thermoplastic resin is extruded onto the surface of a cooling body onto which a film of a liquid heat-transfer medium such as water has previously been formed. Even with this technique disadvantages are encountered particularly at higher cooling drum surface speeds. Further even at low surface speeds of the cooling body, minute surface irregularities or roughness occurs in the sheet. The surface irregularities or roughness can be ignored in ordinary use but if the support is used as a photographic substrate the surface irregularities or roughness is quite disadvantageous.